Detection apparatus for monitoring the rate of change of thickness of a transparent layer using light interference techniques is known. For example, in etching a layer of silicon dioxide on a silicon substrate through a resist window using a reactive gas plasma, a monochromatic light beam is directed onto the surface of the oxide. The light beam, which is conveniently produced by a laser source, is reflected from both the surface of the oxide and from the surface of the underlying opaque silicon. These reflections interfere with one another which varies the light intensity as the thickness of the oxide layer changes. The light intensity changes are sensed by a detector and recorded in the form of a trace. When the trace goes through one period of oscillation, the thickness of the SiO.sub.2 layer has changed by .lambda./2n where .lambda. is the wavelength of the light and n is the refractive index of the SiO.sub.2. This system is essentially a two beam interferometer in which the two beams are parallel to a high degree of accuracy. The small angle between the two beams results in the interference fringe width being practically the width of the laser beam on the detector. This system is, however, not useful in determining the rate of change of thickness of opaque materials such as silicon or metal.
An interferometric system and process has now been devised which can determine the rate of etch of opaque materials.